Are the thermistors still in circuit, or now removed? I assume these are either beads, packaged somehow with short thin wire leads that are soldered into the PCB in some fashion. Thermistors are available in a variety of values, but in this application, they are no doubt NTC (negative temperature coefficient) type, meaning as they are heated, their resistance decreases. So, if you're holding one while trying to get a stable reading, your finger heat is causing them to change. That's what they do. Sit them on the bench, connect your DMM to the leads of each, and let them stablize to the temperature environment. Then, to verify they are working, you can press your finger to them, which should cause the reading to decrease, or even apply heat from your heat gun or soldering iron....just barely, as that is outside their working temperature range (heat sink temperatures range from room temp to around 80-90 deg C).
If these thermistors have failed, you could place a resistance decade box between CR117 cathode and ground (they're using Chassis ground in this section of the schematic). I don't know if the fans run when the thermistors are at room temp or if they start up when the heat sink temp reaches 40 deg C. There is a small chart adjacent to C152 of the fan control circuit schematic, giving you several voltage levels that pertain to the temperature of the heat sink, and the resultant voltage applied to the van, so it definitely is a temp-variable speed control for the fans. You could use this to dial in 1k to start, and verify the fan changes speed as you decrease the resistance. Thermistors are readily available from vendors, both in bead form as well as some packaged.
I had just ordered 10k NTC bead thermistors for a few bucks off ebay, and a package of 10 pcs of aluminum #6-32 x 1/4Â” male-female stud-standoffs, and dropped in a drop of freshly mixed 2-part thermal epoxy into the standoff holes, then dropped the bead probes into it and let them cure overnight. Used to replace a missing thermistor on a BGW GTC Power Amp heat sink, and controls the variable temp fan speed just like your Peavey CS800.